The invention is related to lithium transition metal oxides for use in a rechargeable battery that are doped and coated in a synergistic way to provide excellent battery materials for demanding technologies such as automotive applications.
Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO2-based cathode materials. However, LiCoO2-based cathode materials are expensive and typically have a relatively low capacity of approximately 150 mAh/g.
Alternatives to LiCoO2-based cathode materials include LNMCO type cathode materials. LNMCO means lithium-nickel-manganese-cobalt-oxides. The composition is LiMO2 or Li1+x′M1−x′O2 where M=NixCoyMnzM′m (which is more generally referred to as “NMC”, M′ being one or more dopants). LNMCO has a similar layered crystal structure as LiCoO2 (space group r-3m). The advantage of LNMCO cathodes is the much lower raw material price of the composition M versus pure Co. The addition of Ni gives an increase in discharge capacity, but is limited by a decreasing thermal stability with increasing Ni content. In order to compensate for this problem, Mn is added as a structural stabilizing element, but at the same time some capacity is lost. Typical cathode materials include compositions having a formula LiNi0.5Mn0.3Co0.2O2, LiNi0.6Mn0.2Co0.2O2 or Li1.06M0.94O2, with M=Ni1/3Mn1/3Co1/3O2 (the latter referred to as NMC111). The preparation of LNMCO is in most cases more complex than LiCoO2, because special precursors are needed wherein the transition metal cations are well mixed. Typical precursors are mixed transition metal hydroxides, oxyhydroxides or carbonates.
It is expected that in the future the lithium battery market will be increasingly dominated by automotive applications. Automotive applications require very large batteries that are expensive, and must be produced at the lowest possible cost. A significant fraction of the cost comes from the cathodes, i.e. the positive electrodes. Providing these electrodes by a cheap process can help to lower cost and boost market acceptance. Automotive batteries also need to last for many years. During this time batteries do not always operate. A long battery life is related to two properties: (a) small loss of capacity during storage and (b) high cycle stability.
The automotive market includes different major applications. Batteries for EV (electric vehicles) need to store energy for several hundreds of km of driving range. Thus the cells are very large. Obviously the required discharge rates do not exceed a full discharge within hours. Thus sufficient power density is easily achieved and no special concern is paid to dramatically improve the power performance of the battery. Cathode materials in such batteries need to have a high capacity and a good calendar life.
Contrary to this, HEV (hybrid electric vehicles) have much higher specific power requirements. Electrically assisted accelerations and regenerative braking require that the batteries are discharged or recharged within a couple of seconds. At such high rates the so-called Direct Current Resistance becomes important. DCR is measured by suitable pulse tests of the battery. The measurement of DCR is for example described in “Appendix G, H, I and J of the USABC Electric Vehicle Battery Test Procedures” which can be found at http://www.uscar.org. USABC stands for “US advanced battery consortium” and USCAR stands for “United States Council for Automotive Research”
If the DCR resistance is small, then the charge-discharge cycle is highly efficient; and only a small amount of ohmic heat evolves. To achieve these high power requirements the batteries contain cells with thin electrodes. This allows that (1) Li diffuses over only short distances and (2) current densities (per electrode area) are small, contributing to high power and low DCR resistance. Such high power batteries put severe requirements on the cathode materials: they must be able to sustain very high discharge or charge rates by contributing as little as possible to the overall battery DCR. In the past, it has been a problem to improve the DCR resistance of cathodes. Furthermore, it was a problem to limit the increase of DCR during the long term operation of the battery.
A third type of automotive batteries are batteries for PHEV (plug-in hybrid electric vehicles). The requirements for power are less than HEV but much more than EV type.
The prior art teaches many ways to improve the power properties as well as battery life of cathode materials. However, in many cases these requirements contradict each other. As an example, it is quite generally accepted that a decrease of particle size together with an increase of surface area can increase the power of cathode materials. However, the increase of the surface can have undesirable effects, as one important contribution to a limited battery life are parasitary (undesired) side reactions which happen between the charged cathode and the electrolyte, at the particle/electrolyte interface. The rate of these reactions will increase as the surface area increases. Therefore it is essential to develop cathode materials which have improved power, in particular a low DCR, but without further increasing the surface area of the NMC cathode.
It has been widely reported how doping and coating can help to improve the cycle stability of cathode materials, and ultimately improve the battery life. Unfortunately, many of these approaches cause a deterioration of the power capabilities. In particular doping by Zr, Mg, Al etc., as well as coating by phosphates, fluorites and oxides has been reported, but in many cases and quite generally this results in a lower power performance. The authors believe that this is related to a certain encapsulation effect which happens during doping or coating. The encapsulation prevents or limits the direct contact of the electrolyte with the charged LNMCO cathode surface, but at the same time it becomes more difficult for lithium to penetrate the encapsulating layer. Therefore it is essential to develop improved treated cathode materials which allow to improve the battery life without causing a reduction of power.
The doping of LNMCO materials with Zr is known from U.S. Pat. No. 8,343,662, where Zr is added in order to suppress the decline of the discharge voltage and capacity during charge-discharge cycles, and to improve cycle characteristics. Here a Li precursor and a coprecipitated Ni—Mn—Co hydroxide are mixed with Zr-oxide, and the mixture is heated in air at 1000° C.
In U.S. Pat. No. 7,767,342 it is proposed to dope an oxide of a “dissimilar” element such as aluminum, silicon, titanium, vanadium and others in a lithium transition metal oxide, in order to improve the preservation characteristics of a battery by countering self discharge and increase of the internal resistance. For Ni—Mn—Co complex oxides expensive sintering methods are proposed:
A) mixing a Li-TM (transition metal)—oxide with an oxide of the “dissimilar” element, followed by sintering,
B) mixing Li- and TM-precursors with a “dissimilar” element precursor, followed by sintering in air to oxidize the “dissimilar” element and intermix it in the Li-TM-oxide; or
C) mixing a Li-TM-oxide with a precursor of the “dissimilar” element, followed by sintering under oxidizing conditions.
An example of prior art involving coating followed by a heat treatment is U.S. Pat. No. 8,007,941. A positive active material for a rechargeable lithium battery is disclosed, comprising: a core comprising at least one lithiated compound; and a surface-treatment layer on the core to form the positive active material, the surface-treatment layer comprising a coating material selected from the group consisting of non-lithium hydroxides or non-lithium oxyhydroxides, the coating material comprising a coating element selected from the group consisting of Sn, Ge, Ga, As, Zr, and mixtures thereof, and the coating material having an amorphous form. The material is pretreated by heating to 400° C. to 600° C., followed by heating to 700° C. to 900° C. for 10 to 15 hours to eliminate the carbon of the organic Al carrier.
Both coating with dopants selected from a long list, including Zr, and coating with a metal or metalloid oxide of LNMCO is disclosed in US2011/0076556. However, there is no indication why a dopant should be used, and the metal oxides can be either one of a long list including aluminum-, bismuth-, boron-, zirconium-magnesium-oxides (etc.). Also, the Al2O3 coating is obtained by a high temperature reaction of lithium metal oxide powder whereupon an aluminum hydroxide was precipitated. The extra heating step that is needed after a wet precipitation step to yield the aluminum oxide layer leads to the disadvantageous situation wherein the cathode and the coating layer form an intermediate gradient.
US2002/0192148 discloses a method for forming a lithium metal anode protective layer for a lithium battery having a cathode, an electrolyte, and a lithium metal anode sequentially stacked with the lithium metal anode protective layer between the electrolyte and the lithium metal anode, comprising: activating the surface of the lithium metal anode; and forming a LiF protective layer on the activated surface of the lithium metal anode. US2006/0275667 discloses a cathode active material comprising: complex oxide particle made of an oxide containing at least lithium (Li) and cobalt (Co); and a coating layer which is provided on at least part of the complex oxide particle and is made of an oxide containing lithium and at least one of nickel and manganese. US2005/0227147 discloses a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising: a lithium nickel composite oxide containing lithium, nickel, and at least one metal element other than lithium and nickel; and a layer containing lithium carbonate, aluminum hydroxide, and aluminum oxide, said layer being carried on the surface of said lithium nickel composite oxide
The present invention aims to provide improved lithium transition metal cathode materials for positive electrodes, made by a cheap process, and particularly suitable for automotive battery applications, especially in view of the DCR and the other problems cited before.